Field emission displays are well known and have been proposed as alternatives for conventional cathode-ray tube displays. A conventional field emission display 10 is illustrated in FIG. 1. The conventional field emission display 10 includes a rectangular, generally planar baseplate 12 and a similarly sized, generally planar viewing screen 14 positioned in parallel with the baseplate 12 and spaced a small distance therefrom by a support structure, such as spacers 16. It will be understood by one skilled in the art that the display 10 shown in FIG. 1 is for illustrative purposes only, and is not drawn to scale.
The baseplate 12 includes a substrate 20 of a nonconductive material such as glass, although substrates have also been formed from silicon of one variety or another. In the case of a glass substrate 20, the surface of the substrate 20 facing the display screen 14 is coated with a metal layer 22 such as chromium. As shown in FIG. 1, the metal layer 22 extends only part of the way across the surface of the substrate 20. A layer polysilicon 26 is then deposited on the substrate 20 and at least a portion of the metal layer 22. The polysilicon layer 26 is appropriated doped to be as conductive as reasonably possible. However, as explained below, the resistance of the polysilicon layer 26 is nevertheless higher than desirable.
With further reference to FIG. 1, a large number of conical emitters are formed in the polysilicon layer 26, although only nine emitters 30 are illustrated in FIG. 1. The emitters 30 are generally arranged on the substrate 20 in rows and columns, with the emitters 30 in each column being connected to each other as explained further below. Often, the emitters 30 are arranged in sets, each of which consist of several emitters 30 interconnected to each other. As used herein and in the detailed description of the preferred embodiment and the claims, the term "emitters" encompasses emitter sets.
After the emitters 30 have been formed, a layer of a silicon oxide, such as silicon dioxide 34, is deposited on the polysilicon layer 26. Next, a second layer of polysilicon material 38 is conformably deposited over the oxide layer 34. Finally, a second layer of a metal 42 is deposited over the polysilicon layer 38 to make contact with the polysilicon layer 38. In some circumstances, the metal layer 42 may be deposited on the oxide layer 34 with the polysilicon layer 38 deposited over the metal layer 42. However, in either case, the purpose of the metal layer 42 is to make contact with the polysilicon layer 38. In some cases, the extraction grid may be formed by depositing a layer of metal on the oxide layer 34 in place of the polysilicon layer 38. In such a case, it is unnecessary to use a second metal layer 42 since the metal layer forming the extracting grid serves as the conductor for the extraction grids.
An emitter 30 and its surrounding structure are shown in greater detail in FIG. 2. Openings 50, 52 are formed in the polysilicon layer 38 and the oxide layer 34, respectively, around each emitter 30. The polysilicon layer 38 serves as an extraction grid. When the extraction grid is biased to a positive voltage, for example, 40 volts, and the emitter 30 is at ground, the emitter 30 emits electrons which, as explained below, are attracted to the viewing screen 14 (FIG. 1).
The extraction grids, like the emitters, are generally arranged in rows and columns. However, in the case of the extraction grids, the extraction grids in each row are typically connected to each other and isolated from the extraction grids in the other rows. (It will be understood that the terms "rows" and "columns" are interchangeable in that a row becomes a column by simply rotating the display 90 degrees. Thus, the emitters in each row may be interconnected and the extraction grids in each column may be interconnected.) The emitters 30 in each column are generally connected to each other and isolated from the emitters 30 in the other columns by forming the polysilicon layer 26 and the metal layer 22 in columns that are separated from each other. The metal layer 22 thus makes contact with the polysilicon layer 26 at only the top or bottom of the display. Similarly, the extraction grids in each row are generally connected to each other and isolated from the extraction grids of the other rows by forming the polysilicon layer 38 in rows that are separated apart from each other in the same manner that the polysilicon layer 26 and metal layer 22 are generally formed in columns that are separated from each other. In such cases, the metal layer 42 makes contact with the polysilicon layer 38 only at either the left or right side of the display 10.
With further reference to FIG. 1, the viewing screen 14 includes a transparent panel 60 made from a material such as glass or quartz. The inner surface (i.e., the surface facing the baseplate 12) is coated with a transparent conductive material 62, such as iridium. Finally, the surface of the conductive material 62 is coated with a layer of cathodoluminescent material 64.
In operation, the anode formed by the conductive material 62 is biased to a relatively high voltage, such as 1,000 volts. A column of emitters 30 is biased to a negative voltage or ground potential, and an extraction grid row formed by the polysilicon layer 38 is biased to a positive voltage, such as about 40 volts. The voltage differential between the emitter 30 and an extraction grid at the intersection of the biased column of emitters and row of extraction grids causes the emitter 30 to emit electrons. These electrons are attracted by the positive potential of the anode 62, thereby causing the electrons to strike the cathodoluminescent material 64 and emit light. The light is then viewed through the transparent panel 60.
Although the conventional field emission display shown in FIGS. 1 and 2 is satisfactory in theory, in practice it exhibits a number of serious limitations. First, the resistance of the polysilicon layer 26 is sometimes too high to avoid significant voltage drops as current flows from the emitters 30. As a result, the emitters 30 closer to the conductive material 22 are at a different potential than the emitters 30 farther away from the conductive material 22. The emitters 30 closer to the conductive material 22 then emit more electrons than the emitters 30 farther away from the conductive material 22. As a result, the display is non-uniformly illuminated. While this problem could be solved by extending the conductive material 22 beneath the polysilicon layer 26, thereby providing a uniform resistance between the conductive layer 22 and each emitter 30, doing so would create other problems. More specifically, positioning the conductive layer 22 substantially all of the way across the substrate 20 would result in excessive capacitances between the conductive layer 22 and the polysilicon layer 38 forming the extraction grid. Moreover, the resistance between the conductive layer 22 and each emitter 30 would be too small to provide effective current limiting. It is often desirable to provide a fairly substantial resistance between the conductive layer 22 and the emitters 30 to limit the amount of current that can flow from each emitter 30. Thus, the problem with the prior art approach is not the amount of the resistance between the conductive layer 22 and each emitter, but rather the non-uniformity of this resistance caused by the relatively high resistance of the polysilicon layer 26. Extending the conductive layer 22 beneath the emitters would limit the resistance to the resistance across a very thin layer of polysilicon material which would provide inadequate resistance to effectively limit current.
Still another problem with conventional field emission displays is false emitters that result in short circuits between column lines and row lines. With reference to FIG. 3, the metal layer 22, such as chromium, is normally deposited on the glass substrate 20 by physical vapor deposition or sputtering. Although such a technique generally provides a layer of uniform thickness, at times particles of the metal being deposited can form on the surface of the substrate 20. Also, the metal can be deposited on particles of dirt which find their way onto the surface of the substrate 20. When either of these events occur, a relatively large deposit, known as a false emitter 70, is formed on the substrate 20. The false emitter 70 extends through the first polysilicon layer 26, the oxide layer 34, and makes contact with the second polysilicon layer 38 forming the extraction grids. Under these circumstances, the column of emitters 30 connected to the metal layer 22 will be shorted to the row of extraction grids formed by the portion of the polysilicon layer 38 that is contacted by the false emitter 70.
For the above reasons, practical techniques for performing field emitter displays have resulted in less than ideal field emission displays.